The automotive industry in recent years has recognized the advantages of using electronic fuel management systems to improve vehicle performance over mechanically based fuel control systems. It has been predicted in view of the success of such electronic fuel management systems that in the not too distance future all major automobile manufacturers will turn to electronic control systems for monitoring and controlling the major automobile subsystems.
To increase fuel efficiency and to meet tighter emission requirements such next generation electronic control systems will need more advanced and sophisticated sensors that can be cost effectively manufactured. The microprocessor, which is the heart of such an electronic control system, is capable of executing instructions on the order of magnitude of a one million per second. A need has therefore arisen for mechanically rugged and reliable sensors which have an extremely fast response time. Prior to the present invention, such sensors have been performance limiting factors which have caused delay in the development and implementation of cost effective, integrated vehicle control systems.
In electronic fuel management control systems to provide the required fuel-to-air ratio, it is necessary for the control system to be fed mass airflow rate data. With such data, the controlling microprocessor calculates the amount of fuel needed under the then existing operating conditions to generate a fuel injection control signal.
Prior art mass airflow sensors typically are of the thin-wire or thin-film type. The thin-wire type of sensor is fabricated with a fine resistive wire such as platinum or tungsten wound on a ceramic bobbin. In operation, a predetermined current flows through the wire to heat the resistive wire to a preset temperature. Any airflow alters the rate of heat transfer from the heated wire, thereby causing a wire temperature change (and an attendant change in resistance). Readout circuitry converts this temperature/resistance change into current or voltage changes from which airflow rate may be determined in a manner well known to those skilled in the art.
The thin-wire type of sensor shows critical limitations in electronic fuel management control applications. In this regard, due to the sensor's significant thermal mass, its speed of response is too slow for effective microprocessor-based real time flow control. Also, the use of such thin-wire type sensors renders the overall sensor more bulky than desired. Additionally, the process of fabricating such sensors involves cost inefficient and performance degrading steps. Finally, it is noted that under some noisy environments, the thin-wire type of sensor transmits noise to the external circuit to thereby limit the sensor's flow resolution and accuracy.
An exemplary prior art thin-film type of sensor is the Honeywell microswitch and mass airflow sensor. This sensor includes a "bridge" on the front side of the device which is fabricated by undercutting the wafer substrate from the front side of the wafer.
This "bridge" type of thin-film sensor has a number of disadvantages. The sensor is very sensitive to the direction of airflow over the bridge and the manner in which the sensor device is mounted. Accordingly, it is difficult to achieve precisely reproducible results from sensor to sensor rendering the sensor difficult to calibrate. Furthermore, the bridge structure is not as structurally strong or as rugged as the sensor of the present invention. Additionally, the "bridge" thin-film sensor includes an air channel which is built into the silicon wafer. This tiny air channel (which is required due to the design of the "bridge" type sensor) limits the dynamic range of the sensor such that very high airflow rates cannot be accurately detected.
The present invention is a silicon-based mass airflow sensor which has a high flow sensitivity, high speed of response and sufficient mechanical ruggedness and reliability to be fully compatible with automobile and other industrial fluid flow control systems (e.g., where sensed gas flow rate is used to control gas flow). The mass airflow sensor of the present invention is fabricated using silicon micromachining and integrated circuit techniques which allow the sensor to be reliable, compact and cost-effectively manufactured.
The present invention is a thin-film type of sensor having significant advantages over prior art sensors of the type discussed above. The present invention uses a small, thin dielectric diaphragm providing good thermal isolation for thin-film heating and temperature sensing elements, resulting in high flow sensitivity and low current operation of the heating element. The dielectric diaphragm is bounded by a p-etch-stopped silicon rim. The thermal mass of the diaphragm is so low that the speed of the sensor response to airflow change is much faster than prior art sensor response times. As discussed above, such a fast response time is critically important to real time microprocessor-based airflow control.
In contrast to the "bridge" type sensors, the present invention has a wide dynamic range of airflow which can be accurately detected (in part because it does not require such a small airflow channel). Additionally, the present invention is not nearly as sensitive to airflow direction as the Honeywell sensor.
In the present invention, silicon micromachining techniques are utilized to provide sensor microstructure which is precisely determined. Such micromachining techniques allow the sensor to be made very small while achieving uniform sensor-to-sensor performance.
Silicon micromachining is a process technique which enables precise three dimensional shapes to be formed in silicon. In this regard, control of dimensions may be practically effected from a few microns to a few millimeters with tolerances of less than one micron. The silicon micromachining techniques utilized to fabricate the present invention combines standard semiconductor processes with a variety of etching techniques to batch fabricate sensors that have sensing elements and interface circuitry on the same chip.
Silicon micromachining techniques which may be utilized in fabricating the present invention are described in further detail in above-mentioned co-pending application Ser. No. by Lee entitled "Silicon Based Sensors and Method of Making Same," which application is hereby incorporated by reference herein. Since the fabrication method taught herein is compatible with conventional IC technology the sensor and its readout circuit may be fabricated on a single silicon chip which makes the sensor more reliable, noise-immune and compact.
The present exemplary embodiment is fabricated to incorporate the following important sensor features: (1) a dielectric diaphragm, (2) a p-etch-stopped silicon rim, (3) thin-film resistors, and (4) tapered chip edges. The dielectric diaphragm is formed with thin silicon oxide and silicon nitride in a sandwich structure that provides excellent thermal insulation for the sensor's temperature sensing and heating elements. The diaphragm dimensions including thickness are accurately controlled (to help ensure uniform and reproducible sensor performance) through the use of a heavily-p-doped thin silicon rim. The silicon rim reduces the performance sensitivity to front-backside photolithographic misalignment and wafer thickness variations.